This invention relates to real-time operating systems, and, more particularly, to running a real-time operating system with a general purpose operating system.
General purpose computer operating systems "time-share" programs to optimize use of the machine or response time, or some other mix of objectives. Such programs cannot be used for control of instruments, robots, machine tools, communication devices, or other machinery that requires operation at "hard" real-time, i.e., precise timing where deadlines cannot be missed, because the systems are designed to optimize average performance of application programs at the expense of predictability. Performing the above tasks requires real-time support in the operating system: an ability to schedule tasks at precise intervals, no matter what other system tasks may be active. But real-time support can only be offered if the operating system can ensure (1) low interrupt latency (fast response) and (2) fully pre-emptive task switching. Low interrupt latency means that whenever a hardware device or the clock signals the processor, the operating system will execute a "handler" program within a short and bounded interval of time. Fully preemptive task switching means that whenever a high-priority real-time task is scheduled, it can be executed no matter what other tasks are currently executing.
An exemplary need is a controller for an instrument that measures electrical discharges in thunderstorms. It is desirable to read data from the instruments periodically, buffer and then write the data to disk, generate a graphical display of the data either locally or over the network, and possibly accept data from other instruments over the network. Only the first of these tasks requires hard real-time; the remainder are standard programming tasks for which a general purpose operating system is well suited.
Another exemplary need is the control of a liquid fueled rocket mounted on a test platform. There is a need to sample and display data on numerous channels, update a remote real-time display, accept emergency shutdown commands, and perform routing control operations. Again, most of the requirements are for conventional operating systems services, but there are hard real-time components that need reasonably precise scheduling. For example, the shutdown sequence must be precisely timed and cannot be delayed by lower priority tasks without spectacular and unwelcome results.
It is possible to design a special purpose operating system to support real-time, but this is an enormously complex, expensive and error-prone process that produces a system that needs a large continuing investment to remain current. In addition, there is substantial ongoing development effort and it is desirable to maintain compatibility with these developments, which are generally done by others.
Accordingly, it is an object of the present invention to operate a real-time operating system, or executive, and retain the capabilities offered by a general purpose operating system.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.